As well known, a glass bulb in a cathode ray tube (CRT) used in a TV set or a computer monitor basically includes a panel for displaying picture images, a conical funnel sealed to the back of the panel and a cylindrical neck integrally connected to an apex portion of the conical funnel. The panel, the funnel and the neck are made of glass, wherein particularly the panel and the funnel are formed of predetermined dimensions and shapes by press forming a glass gob.
Referring to FIG. 1, there is illustrated a cross sectional view of a conventional glass bulb 10. A conventional flat panel 20 of the glass bulb 10 is provided with a face portion 21 whose inner surface is covered with an array of dots of fluorescent material (not shown) to display picture images; a skirt portion 23 extending backward from a perimeter of the face portion 21 and having a seal edge 22 on its back edge; and a blend round portion (or corner portion) 24 integrally joining the face portion 21 to the skirt portion 23. A funnel 30 of the glass bulb 10 can be divided into a body portion 32, i.e., a fore part thereof, having a seal edge 31 connected to the seal edge 22 of the skirt portion 23; and a yoke portion 33, i.e., a back part thereof, extending backward from the body portion 32. And a neck 40 of the glass bulb 10 is connected to the yoke portion 33 of the funnel 30. A tube axis 11 passes through the center of the face portion 21 and coincides with an axis of the neck 40. Placed by way of the so-called “shrinkage fit” scheme around the outer periphery of the skirt portion 23 is a metallic implosion-proof band 50, which strengthens the bulb 10 against tensile stress induced in the blend round portion 24 and the skirt portion 23 by evacuating the inner space of the bulb 10, so that fragments of the glass can be prevented from flying away when the panel 20 is broken or exploded.
Referring to FIG. 2, there is illustrated a schematic cross-sectional view of a mold set 60 for forming the panel 20. The mold set 60 is provided with a bottom mold 62 in which a cavity 61 is formed; a middle mold (or shell) 63, for forming the skirt portion 23 and the seal edge 22, which is fitted on top of the bottom mold 62; and an upper mold 64 (or plunger) which presses a glass gob loaded in the cavity 61 of the bottom mold 62 to form the panel 20. The upper mold 62 mold 64 is connected to a press ram 65, so that it can be lifted or lowered by the ram 65 so as to press the glass gob loaded in the cavity 61 of the bottom mold 62 to form the panel 20. There exists a parting line 66 between the bottom mold 62 and the middle mold 63. Therefore, when the panel 20 is formed in the mold set 60 as shown in FIG. 1, a mold match line 25, which is a flash made by the parting line 66, is formed on the outer periphery of the skirt portion 23 near the face portion 21. The peripheral length of the mold match line 25 represents the maximum peripheral length of the panel 20. And, in general, the position of the parting line 66 and thus the position of the mold match line 25 are set near the face portion 21 rather than the seal edge 22 in order to ease the extraction of the molded panel 20 from the bottom mold 62.
With reference to FIG. 1, the implosion-proof band 50 is installed in order to not only suppress from the flat panel 20 to the funnel 30 the propagation of waves and cracks incurred by an impact applied to the flat panel 20, but also reduce the vacuum stress of the bulb 10. And because the glass bulbs are getting thinner to lighten a CRT, the implosion-proof band is required to reduce comparatively more vacuum stress of the glass bulb to prevent implosion of the glass bulb. In order to do so, the following two schemes have been conventionally utilized: reducing the inner peripheral length of the pre-expanded implosion-proof band and moving the implosion-proof band toward the face portion. The first scheme relates only to a configuration of the implosion-proof band itself and hence is excluded from this discussion. The second scheme is to move the implosion-proof band toward the face portion, i.e., to increase the height of the implosion-proof band, but it has been carried out without changing the location of the mold match line. Therefore, if the implosion-proof band is moved above a certain height, it cannot reduce the vacuum stress anymore. That is, the implosion-proof band disposed above the certain height does not effectively clamp or compress a maximum peripheral length part of the flat panel, i.e., the mold match line.
Accordingly, in order to solve such a drawback, the forming position of the mold match line together with the installing position of the implosion-proof band need to be moved near the face portion while a predetermined distance between the mold match line and the upper edge of the implosion-proof band is maintained.
However, in a case where the mold match line is formed too close to the face portion, the outer contour of the blend round portion becomes sharp (or a sharp round) after a polishing process for removing defects from the outer surface of the face portion. This sharp round easily cracks and breaks even in a case where it is subject to a weak exterior impact.
Further, in a case where the mold match line is formed near the face portion of the flat panel, the skirt portion, which is not completely hardened, tends to bend inwardly when the upper mold is extracted from the first mold.